1. Field of the Invention
The present invention relates to processes for producing (meth)acrylic acid. Particularly, the present invention relates to processes for producing (meth)acrylic acid of a high purity constantly by continuous distillation and purification of crude (meth)acrylic acid obtained by vapor-phase catalytic oxidation.
In this specification, “(meth)acrylic acid” is a general term for acrylic acid and methacrylic acid, and it may be either one or both of them.
2. Discussion of Background
Acrylic acid is industrially important as a raw material for a resin with high water absorbability and further, as a raw material for various acrylic esters. Recently, it is produced by a vapor-phase catalytic oxidation reaction of propylene.
In processes to obtain acrylic acid by oxidizing propylene, conditions for oxidation to acrolein and oxidation to acrylic acid in the next step are different. Accordingly, there is a two step oxidation process in which the respective steps are carried out in separate reactors and a process in which oxidization is proceeded directly to acrylic acid in one step. An acrylic acid-containing gas obtained by such a vapor-phase catalytic oxidation reaction, is contacted with water in a collection column (condensation column) to obtain an aqueous acrylic acid solution, which is extracted in an extraction column by adding a suitable extraction solvent, whereupon the extraction solvent is separated in a solvent separation column. Then, acetic acid is separated in an acetic acid separation column, and further, a byproduct is separated in a fractionating column to obtain a purified product of acrylic acid.
Further, in recent years, instead of a solvent extraction method wherein recovery of acrylic acid from the above aqueous acrylic acid solution is carried out by means of an extraction solvent, an azeotropic separation method is carried out wherein distillation is carried out by means of water and an azeotropic solvent, as show in the after-mentioned FIGS. 1 and 2, so that from the top of an azeotropic separation column, an azeotropic mixture comprising water and the azeotropic solvent, is distilled, and from the bottom, acrylic acid is recovered.
Acrylic acid has a very high polymerizability, and a solid substance is frequently formed by a polymerization reaction in the step of distillation and purification, whereby a trouble such as clogging of devices takes place, which tends to hinder a constant continuous operation. Therefore, by adding a polymerization inhibiter (such as hydroquinone, phenothiazine, copper dithiocarbamate, an n-oxyl compound or air) and by decreasing hot sections and retention sections to the minimum, ingenuities in the operation and device are carried out to suppress such an undesirable polymerization reaction. (Acrylic Acid and Polymer Thereof [I]: published by Shokodo Co. Ltd., JP-A-7-252477, JP-A-7-228548, JP-A-10-175912, JP-A-8-239341)
However, it is insufficient to suppress the polymerization in the purification step of acrylic acid by such a countermeasure alone. Accordingly, it is desired to develop a technique, whereby polymerization of acrylic acid can more certainly be prevented so as to accomplish a constant continuous operation.
Further, heretofore, in order to prevent the polymerization of acrylic acid, various studies have been made as mentioned above. However, it has not been reported that an adduct of two molecules of (meth)acrylic acid, such as a β-acryloxypropionic acid or a β-methacryloxyisobutyric acid, of which the concentration is to be reduced by the present invention, is involved in a polymerization trouble in the step of distillation and purification of (meth)acrylic acid.
It is an object of the present invention to provide a process for producing (meth)acrylic acid, whereby the undesired polymerization reaction of (meth)acrylic acid in a step of distillation and purification of (meth)acrylic acid is suppressed, a trouble such as clogging of devices is avoided, and a constant continuous operation can be carried out for a long period of time.